Disclosed herein are the design, synthesis, and biological evaluation of a novel inhibitor of HIV-1 protease, as well as aspects of the X-ray crystal structure of the novel inhibitor bound with HIV-1 protease. Various C3-functionalized cyclopentanyltetrahydrofurans (Cp-THF) were designed to interact with the flap Gly48 carbonyl or amide NH in the S2-subsite of the HIV-1 protease. The anti-HIV activity of these functionalized ligands combined with several hydroxyethyl sulfonamide isosteres is described. Inhibitor 25, containing a 3(R)-hydroxyl group on the Cp-THF core, displayed potent enzyme inhibitory and antiviral activity. A preference of the 3(R)-configuration over the corresponding 3(S)-derivative is described. Inhibitor 25 exhibited potent activity against a panel of multidrug-resistant HIV-1 variants.
Human immunodeficiency virus 1 (HIV-1) protease inhibitors are critical components of antiretroviral therapies. However, the emergence of drug resistance has raised serious questions about long-term treatment options. Structure-based design of inhibitors targeting the protein-backbone has led to the discovery of a variety of novel HIV-1 protease inhibitors (PIs) with broad-spectrum activity against multidrug-resistant HIV-1 variants. One of these inhibitors, darunavir (1) was approved by the FDA for the treatment of HIV/AIDS patients. Inhibitor design strategies have focused on maximizing active site interactions with the protease, particularly by promoting extensive hydrogen bonding interactions with backbone atoms throughout the active site.
Several potent inhibitors incorporating a stereochemically defined (3aS,5R,6aR)-hexahydro-2H-cyclopenta[b]furan-5yl (Cp-THF) as the P2-ligand with a modified hydroxyethylsulfonamide isostere have been described recently. The X-ray crystal structure of 3-bound HIV-1 protease revealed the formation of an extensive hydrogen-bonding network between the inhibitor and the active site. Incorporation of a stereochemically defined lactam at the P1′-position to further enhance backbone interactions has also been described. Interestingly, the resulting inhibitor 4 retained full potency against a range of multidrug-resistant HIV-1 variants. The X-ray structural studies of 4-bound HIV-1 protease evidenced enhanced backbone interactions with the Gly27′ carbonyl at the S1′-subsite. Without being bound by theory, it is believed that the Cp-THF ligand fits within the S2-subsite and the cyclic ether oxygen is involved in a close hydrogen bonding interaction with the backbone NH of Asp29 (2.8 Å). Several structural modifications of the Cp-THF ligand to further improve ligand binding, particularly hydrogen bonding ability, in the S2-subsite are described herein. The X-ray structure of 3-bound HIV-1 protease indicated that the C3 methylene of the Cp-THF is in close proximity to the protease flap region. The X-ray data suggested a weak C3-H . . . O interaction with the Gly48 backbone carbonyl group. Introduction of polar substituents at the C3 position which may lead to additional interactions of the Cp-THF ligand with the protease flap residues are described herein. Without being bound by theory, it is believed that an inhibitor which makes tight interactions with the protease flap region could conceivably delay its dissociation via opening of the flaps. The design, synthesis, and biological evaluation of a series of protease inhibitors that incorporate stereochemically defined functionality at the C3 position of the Cp-THF-ligand are described. Inhibitor 25, incorporating a 3(R)-hydroxyl group was one of the most potent protease inhibitor (PI) (Ki=5 pM; antiviral IC50=2.9 nM). This inhibitor also maintained excellent potency against a range of multidrug-resistant HIV-1 variants.

Structures of Potent HIV-1 Protease Inhibitors 1-4, and 25
The HIV aspartic protease plays a central role in the life cycle of the virus. It cleaves the virally encoded polyprotein and promotes the release of all essential viral enzymes and structural proteins necessary for the formation of new infective virions. Inhibition of this enzyme logically appeared as a viable strategy for the treatment of HIV-1 infection and AIDS. The development of HIV-1 protease inhibitors and their combination with reverse transcriptase inhibitors in Highly Active Antiretroviral Therapies (HAART) have drastically changed the outcome of the AIDS epidemic. These therapies effectively suppressed virus replication and increased CD4 cells counts in patients, and ultimately reduced HIV-AIDS-associated morbidity and mortality. Despite these advances, several limitations are still severely hampering current HAART regimens. Poor drug bioavailability, heavy pill burden, debilitating side effects, and high treatment cost remain serious drawbacks. However, the fast emergence of drug-resistance certainly remains the most distressing issue in the management of HIV-1 infection. Through spontaneous viral mutations, or recombination between mutated viral strains, occurrence of drug-resistance and cross-resistance quickly compromise long-term treatment options. As a result, the development of new protease inhibitors with broad-spectrum activity, less toxicity, and improved bioavailability remains a critical objective.
The design of several novel HIV-1 proteases inhibitors (PIs) with broad-spectrum activity against multidrug-resistant HIV-1 variants has been described. These include darunavir (DRV, TMC-114, 1), GRL-0476A (2), or inhibitors 3 and 4. DRV has exhibited exceptional activity against both wild-type and multi-drug resistant HIV-1 viruses. This inhibitor also showed excellent resilience towards the development of drug-induced resistance. Darunavir (1, DRV) was approved by the FDA in 2006 for the treatment of antiretroviral-experienced patients, and in 2008, extended approval was granted for treatment-naïve adults and for use in pediatrics.
Without being bound by theory, it is believed that inhibitor design relying on maximizing interactions with the protease active site, and particularly with the protease backbone atoms provides a strategy for minimizing the development of drug resistance. Several studies have noted a minimal distortion of the protease backbone is associated with mutations, it is believed that an inhibitor that strongly binds to the protease backbone atoms would retain its affinity and potency with mutant proteases. The close hydrogen bonding interactions observed between DRV, especially through its bis-THF P2 ligand, and the protease's backbone residues might explain DRV's marked potency and resistance profile.

Structures of Potent HIV-1 Protease Inhibitors 1-5
Inhibitors 3 and 4 have also displayed excellent activities against multidrug-resistant HIV-1 variants. These inhibitors possess a stereochemically-defined (3aS,5R,6aR)-hexahydro-2H-cyclopenta[b]furan-5-yl (Cp-THF) as the P2 ligand used in combination with a hydroxyethyl sulfonamide isostere. Similar to DRV, crystal structure analysis of the 4-bound HIV protease complex revealed an extensive hydrogen-bonding network between the inhibitor and the enzyme binding site. The Cp-THF ligand was observed to tightly fit within the S2 subsite of the protease, with the cyclic ether oxygen at close hydrogen bonding distance (2.8 Å) to the Asp29 NH backbone bond. Structure activity studies on this ligand moiety showed an important influence of the cyclic ether oxygen on the PI activity. The Cp-THF ligand was discovered to provide an important structural framework for the design of potent HIV PIs.
The effect of structural modifications of the Cp-THF ligand on inhibitor activity, by introducing additional functionalities on that ligand are described herein. It has been discovered that functional group insertion at the 3-position of the ligand yields active inhibitors. X-Ray crystal structure analysis of the 3-bound protease showed that the C3 methylene of the Cp-THF is near the flap region of the protease. Particularly, a weak C3-H . . . O interaction with the Gly-48 backbone carbonyl bond was noted previously. It was found that introducing polar substituents on the C3 position of the Cp-THF favors additional interactions with protease flap residues and enhances the binding affinity of the inhibitor. Without being bound by theory, it is believed that an inhibitor having close binding interactions with the flap region of the protease can delay its kinetics of dissociation by inhibiting the opening of the flap. Several inhibitors containing novel stereochemically-defined spirocyclic P2-ligands based on the Cp-THF framework have recently been reported. Stepwise modification of the spirocylic moiety led to inhibitors exhibiting high inhibitory potency and good antiviral activity. Spirocyclic oxazolidinone-containing inhibitor 5 showed the highest potency and activity in that group of compounds. It has been discovered that addition of polar functional groups, hydrogen bond donor or acceptor, at the 3-position of the Cp-THF, results improved inhibitor activity.
Herein, is reported the design, synthesis, and biological evaluation of a new series of inhibitors that contain Cp-THF ligand analogs with various functionalities on the C3 position. Inhibitor 25, with a (3S)-hydroxyl group, has potent PI (5 pM) and antiviral activity (IC50=2.9 nM). Structure-activity studies revealed the benefit of introducing a hydrogen bonding donor like hydroxy at the C3 position. When evaluated against a panel of multidrug-resistant HIV-1 viruses, 25 retained high potency against all mutant strains. An X-Ray crystal structure of 25-bound HIV protease provided showed several of the interactions of the inhibitor with the protease active site as well as with the flap region of the protease.
Part B
Several illustrative embodiments of the invention are described by the following enumerated clauses:
1. A compound having the formula
or a pharmaceutically acceptable salt, isomer, mixture of isomers, crystalline form, non crystalline form, hydrate, or solvate thereof; wherein
m is 1 or 2; n is 1 or 2;
X1 is C(O), S(O), S(O)2, HNS(O)2, HNC(O), or C(O)NH;
R1 and R2 are in each instance independently selected from the group consisting of hydrogen, and a prodrug forming group;
RA represents from 0 to 4 substituents independently selected in each instance from the group consisting of hydroxy, alkyl, heteroalkyl, alkyloxy, heteroalkyloxy, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted; or RA represents from 2 to 4 substituents where two of the substituents are vicinal substituents and taken with the attached carbons form a carbocycle or heterocycle; and the remaining substituents, if present, are independently selected in each instance from the group consisting of alkyl, heteroalkyl, alkyloxy, heteroalkyloxy, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted;
RB represents from 0 to 4 substituents independently selected in each instance from the group consisting of amino, alkyl, heteroalkyl, alkyloxy, heteroalkyloxy, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted; or RB represents from 2 to 4 substituents where two of the substituents are vicinal substituents and taken with the attached carbons form a carbocycle or heterocycle; and the remaining substituents, if present, are independently selected in each instance from the group consisting of alkyl, heteroalkyl, alkyloxy, heteroalkyloxy, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted;
R3 is alkyl, cycloalkyl, arylalkyl, or heteroarylalkyl, each of which is optionally substituted;
R4 and R5 are independently selected in each instance from hydrogen, halogen, oxo, hydroxy, or the group consisting of alkyl, heteroalkyl, alkyloxy, hydroxyalkyl, acyloxy, —OC(O)-amino, acyl, amino, sulfonylamino, acylamino, alkyloxy-C(O)-amino, and alkoxyamino, each of which is optionally substituted, where at least one of R4 or R5 is not hydrogen.
2. The compound of clause 1 wherein m and n are not both 2.
3. The compound of clause 1 or 2 wherein m and n are 1.
4. The compound of any one of the preceding clauses wherein the compound has the formula

5. The compound of any one of the preceding clauses wherein the compound has the formula

6. The compound of any one of clauses 1 to 5 wherein the compound has the formula

7. The compound of any one of clauses 1 to 5 wherein the compound has the formula

7.1 The compound of any one of clauses 1 to 7 wherein X1 is SO2; R3 is iso-butyl; and R1, R2, and R4 are hydrogen.
8. The compound of any one of clauses 1 to 7 wherein R4 is alkyl or an oxygen containing substituent.
9. The compound of any one of the preceding clauses wherein R5 is alkyl or an oxygen containing substituent.
10. The compound of any one of the preceding clauses wherein RB is a substituent at the 4-position selected from the group consisting of amino, alkyloxy, and hydroxyalkyl.
11. The compound of any one of the preceding clauses wherein RB is NH2, CH3O, or CH2OH.
12. The compound of any one of the preceding clauses wherein X1 is S(O)2.
13. The compound of any one of the preceding clauses wherein R3 is branched alkyl.
14. The compound of any one of the preceding clauses wherein R3 is iso-butyl.
15. The compound of any one of the preceding clauses wherein RA is absent.
16. The compound of any one the preceding clauses wherein R1 is hydrogen.
17. The compound of any one the preceding clauses wherein R2 is hydrogen.
18. The compound of any one of the preceding clauses wherein R4 is hydrogen.
18.1 The compound of any one of the preceding clauses wherein R5 is selected from the group consisting of hydroxy, alkyloxy, —OC(O)-amino, acylamino, alkyloxy-C(O)-amino, and sulfonylamino.
19. The compound of any one of the preceding clauses wherein R5 is selected from the group consisting of hydroxy, alkyloxy, —OC(O)-amino, acylamino, and sulfonylamino.
19.1 The compound of any one of the preceding clauses wherein R5 is hydroxy, methoxy, CH3OC(O)NH, or acetamido.
19.2 The compound of any one of the preceding clauses wherein R5 is CH3OC(O)NH.
20. The compound of any one of the preceding clauses wherein R5 is hydroxy, methoxy, or acetamido.
21. The compound of any one of the preceding clauses wherein the compound binds to HIV-1 protease, where the binding includes a direct or indirect interaction between the compound and one or more amino acid residues in the protease selected from the group consisting of Asp29, Asp30, and Gly48.
22. The compound of clause 21 wherein the compound binds directly to the one or more amino acid residues.
23. The compound of clause 21 wherein the compound binds indirectly to the one or more amino acid residues, wherein the indirect binding includes an intervening group that binds to the amino acid residue and the compound, where the intervening group is independently selected in each instance from the group consisting of a water molecule, a divalent metal ion, a phosphate ion, a sulfate ion, an ammonium ion, and a monovalent metal ion.
24. The compound of clause 23 wherein the intervening group is a water molecule.
25. A pharmaceutical composition comprising one or more compounds of any one of the preceding clauses.
26. The pharmaceutical composition of clause 25 further comprising one or more carriers, diluents, or excipients, or a combination thereof.
27. A method for treating HIV in a patient, the method comprising the step of administering to the patient a therapeutically effective amount of one or more compounds of any one of clauses 1 to 24, or the composition of clauses 25 to 26.
In another embodiment, a compound having the following formula is described.
                R1=alkyl, hydroxy, alkoxy, OCH2CH2O-alkyl, amine, substituted amine, urethane, amide, aminosulfonamide        R1=alkyl, hydroxy, alkoxy, amine, substituted amine, urethane, amide, aminosulfonamide n=1, 2, 3        R3=alkyl, OMe, OET, O-heteroalkyl, OCH2-CH2-morpholine, OCH2-oxazole,        R4=alkyl, heteroalkyl, hydroxyalkyl, alkoxy, amine, substituted amine, urethane, amide, aminosulfonamide        X=SO2, NHSO2-, NHCO, C═ONH        Y=OMe, NH2, CH2OMe, CH2NH2, substituted amine, urethane, amide, aminosulfonamide        Z=H, OMe, NH2, CH2OMe, CH2NH2, substituted amine, urethane, amide, aminosulfonamide                    Y—Z=4,5,6-membered ring/substituted containing heteroatoms                        
As used herein, the term “alkyl” includes a chain of carbon atoms, which is optionally branched. As used herein, the term “alkenyl” and “alkynyl” includes a chain of carbon atoms, which is optionally branched, and includes at least one double bond or triple bond, respectively. It is to be understood that alkynyl may also include one or more double bonds. It is to be further understood that in certain embodiments, alkyl is advantageously of limited length, including C1-C24, C1-C12, C1-C8, C1-C6, and C1-C4. It is to be further understood that in certain embodiments alkenyl and/or alkynyl may each be advantageously of limited length, including C2-C24, C2-C12, C2-C8, C2-C6, and C2-C4. It is appreciated herein that shorter alkyl, alkenyl, and/or alkynyl groups may add less lipophilicity to the compound and accordingly will have different pharmacokinetic behavior. Illustrative alkyl groups are, but not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, 2-pentyl, 3-pentyl, neopentyl, hexyl, heptyl, octyl and the like.
As used herein, the term “cycloalkyl” includes a chain of carbon atoms, which is optionally branched, where at least a portion of the chain in cyclic. It is to be understood that cycloalkylalkyl is a subset of cycloalkyl. It is to be understood that cycloalkyl may be polycyclic. Illustrative cycloalkyl include, but are not limited to, cyclopropyl, cyclopentyl, cyclohexyl, 2-methylcyclopropyl, cyclopentyleth-2-yl, adamantyl, and the like. As used herein, the term “cycloalkenyl” includes a chain of carbon atoms, which is optionally branched, and includes at least one double bond, where at least a portion of the chain in cyclic. It is to be understood that the one or more double bonds may be in the cyclic portion of cycloalkenyl and/or the non-cyclic portion of cycloalkenyl. It is to be understood that cycloalkenylalkyl and cycloalkylalkenyl are each subsets of cycloalkenyl. It is to be understood that cycloalkyl may be polycyclic. Illustrative cycloalkenyl include, but are not limited to, cyclopentenyl, cyclohexylethen-2-yl, cycloheptenylpropenyl, and the like. It is to be further understood that chain forming cycloalkyl and/or cycloalkenyl is advantageously of limited length, including C3-C24, C3-C12, C3-C8, C3-C6, and C5-C6. It is appreciated herein that shorter alkyl and/or alkenyl chains forming cycloalkyl and/or cycloalkenyl, respectively, may add less lipophilicity to the compound and accordingly will have different pharmacokinetic behavior.
As used herein, the term “heteroalkyl” includes a chain of atoms that includes both carbon and at least one heteroatom, and is optionally branched. Illustrative heteroatoms include nitrogen, oxygen, and sulfur. In certain variations, illustrative heteroatoms also include phosphorus, and selenium. As used herein, the term “cycloheteroalkyl” including heterocyclyl and heterocycle, includes a chain of atoms that includes both carbon and at least one heteroatom, such as heteroalkyl, and is optionally branched, where at least a portion of the chain is cyclic. Illustrative heteroatoms include nitrogen, oxygen, and sulfur. In certain variations, illustrative heteroatoms also include phosphorus, and selenium. Illustrative cycloheteroalkyl include, but are not limited to, tetrahydrofuryl, pyrrolidinyl, tetrahydropyranyl, piperidinyl, morpholinyl, piperazinyl, homopiperazinyl, quinuclidinyl, and the like.
As used herein, the term “aryl” includes monocyclic and polycyclic aromatic carbocyclic groups, each of which may be optionally substituted. Illustrative aromatic carbocyclic groups described herein include, but are not limited to, phenyl, naphthyl, and the like. As used herein, the term “heteroaryl” includes aromatic heterocyclic groups, each of which may be optionally substituted. Illustrative aromatic heterocyclic groups include, but are not limited to, pyridinyl, pyrimidinyl, pyrazinyl, triazinyl, tetrazinyl, quinolinyl, quinazolinyl, quinoxalinyl, thienyl, pyrazolyl, imidazolyl, oxazolyl, thiazolyl, isoxazolyl, isothiazolyl, oxadiazolyl, thiadiazolyl, triazolyl, benzimidazolyl, benzoxazolyl, benzthiazolyl, benzisoxazolyl, benzisothiazolyl, and the like.
As used herein, the term “amino” includes the group NH2, alkylamino, and dialkylamino, where the two alkyl groups in dialkylamino may be the same or different, i.e. alkylalkylamino. Illustratively, amino includes methylamino, ethylamino, dimethylamino, methylethylamino, and the like. In addition, it is to be understood that when amino modifies or is modified by another term, such as aminoalkyl, or acylamino, the above variations of the term amino are included therein. Illustratively, aminoalkyl includes H2N-alkyl, methylaminoalkyl, ethylaminoalkyl, dimethylaminoalkyl, methylethylaminoalkyl, and the like. Illustratively, acylamino includes acylmethylamino, acylethylamino, and the like.
As used herein, the term “amino and derivatives thereof” includes amino as described herein, and alkylamino, alkenylamino, alkynylamino, heteroalkylamino, heteroalkenylamino, heteroalkynylamino, cycloalkylamino, cycloalkenylamino, cycloheteroalkylamino, cycloheteroalkenylamino, arylamino, arylalkylamino, arylalkenylamino, arylalkynylamino, heteroarylamino, heteroarylalkylamino, heteroarylalkenylamino, heteroarylalkynylamino, acylamino, and the like, each of which is optionally substituted. The term “amino derivative” also includes urea, carbamate, and the like.
As used herein, the term “hydroxy and derivatives thereof” includes OH, and alkyloxy, alkenyloxy, alkynyloxy, heteroalkyloxy, heteroalkenyloxy, heteroalkynyloxy, cycloalkyloxy, cycloalkenyloxy, cycloheteroalkyloxy, cycloheteroalkenyloxy, aryloxy, arylalkyloxy, arylalkenyloxy, arylalkynyloxy, heteroaryloxy, heteroarylalkyloxy, heteroarylalkenyloxy, heteroarylalkynyloxy, acyloxy, and the like, each of which is optionally substituted. The term “hydroxy derivative” also includes carbamate, and the like.
As used herein, the term “acyl” includes formyl, and alkylcarbonyl, alkenylcarbonyl, alkynylcarbonyl, heteroalkylcarbonyl, heteroalkenylcarbonyl, heteroalkynylcarbonyl, cycloalkylcarbonyl, cycloalkenylcarbonyl, cycloheteroalkylcarbonyl, cycloheteroalkenylcarbonyl, arylcarbonyl, arylalkylcarbonyl, arylalkenylcarbonyl, arylalkynylcarbonyl, heteroarylcarbonyl, heteroarylalkylcarbonyl, heteroarylalkenylcarbonyl, heteroarylalkynylcarbonyl, acylcarbonyl, and the like, each of which is optionally substituted.
The term “optionally substituted” as used herein includes the replacement of hydrogen atoms with other functional groups on the radical that is optionally substituted. Such other functional groups illustratively include, but are not limited to, amino, hydroxyl, halo, thiol, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof, and the like. Illustratively, any of amino, hydroxyl, thiol, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, and/or sulfonic acid is optionally substituted.
As used herein, the terms “optionally substituted aryl” and “optionally substituted heteroaryl” include the replacement of hydrogen atoms with other functional groups on the aryl or heteroaryl that is optionally substituted. Such other functional groups illustratively include, but are not limited to, amino, hydroxy, halo, thio, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, nitro, sulfonic acids and derivatives thereof, carboxylic acids and derivatives thereof, and the like. Illustratively, any of amino, hydroxy, thio, alkyl, haloalkyl, heteroalkyl, aryl, arylalkyl, arylheteroalkyl, heteroaryl, heteroarylalkyl, heteroarylheteroalkyl, and/or sulfonic acid is optionally substituted.
Results and Discussions
Inhibitors were designed to make additional interactions in the S2-subsite of the protease, especially with the Gly48 backbone atoms in the flap region of the enzyme. All inhibitors in Table 1 were first evaluated in the enzyme inhibitory assay developed by Toth and Marshall (Toth and Marshall, Int. J. Pept. Protein Res. 1990, 36, 544-550). Inhibitors that exhibited potent Ki values were subsequently evaluated for in vitro antiviral assays. As can be seen in the Table 1, all inhibitors displayed sub-nanomolar to low picomolar inhibitory potencies. Inhibitor 22, with a 3-(S)-hydroxy group on the Cp-THF, was significantly more potent than the keto derivative 19a (entries 1 and 2). The 3-(S)-hydroxy Cp-THF ligand was also investigated in combination with other sulfonamide substituents. Inhibitor 123 with a p-amino sulfonamide as the P2′-ligand displayed impressive inhibitory potency, however, its antiviral activity was 3-fold lower than 22. Inhibitor 23 with a p-hydroxymethyl sulfonamide as the P2′-ligand has shown a reduction in potency. Inhibitor 19b which contains a 3-(S)-methoxy substituent, exhibited a significant loss of potency and a near 5-fold loss of antiviral activity compared to 22. Inhibitor 25 with 3-(R) configuration displayed an impressive enzyme inhibitory and antiviral activity. Inhibitor 26 with the 4-amino sulfonamide isostere also showed comparable enzyme inhibitory activity. Inhibitor 19d, with a 3-(R)-methoxy group, also exhibited comparable inhibitory potency.
In order to probe the importance of the C3-hydroxyloxygen on the Cp-THF ring, inhibitors 19e and 19f, with a methyl group in place of a C3-oxygen were synthesized. Inhibitor 19e, which contains a 3(S)-methyl group showed a significant reduction in potency compared to 22. Similarly, inhibitor 19f with a 3(R)-methyl group has shown a reduction in enzyme Ki value compared to the corresponding hydroxy derivative 25. An amine substitution on the Cp-THF ligand is also described. Inhibitor 24c with C3-dimethylamine exhibited lower enzyme inhibitory potency compared to the corresponding hydroxyl or methoxyl derivatives.
Inhibitors 22 with a 3(S)-hydroxyl group and 25 with a 3(R)-hydroxyl group on the Cp-THF ligand were tested against a panel of multidrug-resistant HIV-1 variants. Their antiviral activity was compared against other clinically available PIs including APV and DRV. The results are shown in Table 2. All inhibitors in Table 2 exhibited high antiviral activity against the wild-type HIV-1 laboratory strain, HIV-1ERS104pre, isolated from a drug-naïve patient. Compound 25 provided the most potent activity with an IC50 of 2.9 nM, comparable to that of DRV. When tested against various multidrug-resistant HIV-1 strains, the IC50 values of inhibitor 25 remained in the low nanomolar values (2.9-29 nM), and the relative change in IC50 did not exceed 10-fold. Isomeric inhibitor 22 displayed lower activity against the wild-type viral strain (IC50=20 nM). It also exhibited a much larger relative IC50 change, and in some cases, only low activity against multidrug-resistant HIV-1 variants. The contrast in antiviral activity of 22 compared to 25 underlines the importance of the stereochemistry at the 3-position of the Cp-THF ligand. Inhibitor 25 displayed a superior profile compared to another approved PI, APV. Overall, inhibitor 25 maintained high potency against all tested multidrug-resistant HIV-1 strains. It showed comparable activity to DRV, which is the leading PI for the treatment of multidrug resistant HIV infection.
An X-ray crystal structure of the inhibitor-bound wild-type HIV-1 protease, refined to a 1.45 Å resolution has been determined. The protease dimer binds with the inhibitor in two orientations related by a 180° rotation with a 0.55/0.45 ratio. The protease backbone structure showed a very low RMS deviation of 0.15 Å for all Ca atoms compared to protease complexes of 2-bound inhibitor and darunavir. The inhibitor binds with extensive interactions across the S2 to ST ligands with the protease atoms, and most notably displays favorable polar interactions including hydrogen bonds, weaker C—H . . . O and C—H . . . pi interactions. The central hydroxyl group forms hydrogen bonds with the side chain carboxylate oxygen atoms of the catalytic Asp25 and Asp25′ residues. The inhibitor hydrogen bonds with the protease backbone atoms of the amide of Asp30′, the carbonyl oxygen of Gly27, and forms water-mediated interactions with the amides of Ile50 and Ile50′, which are conserved in the majority of protease complexes with inhibitors or substrate analogs. The inhibitor interactions with atoms in the binding cavity resemble those of darunavir and TMC-126 with the exception of the interactions of the new P2-ligand that replaces the bis-THF group. The 3-(R)-hydroxyl of the Cp-THF ligand extends towards the flap region and forms a new water-mediated hydrogen bond interaction with the backbone amide NH of Gly48, with interatomic distances of 2.5 Å, 3.1 Å for the major inhibitor orientation or 2.7 Å, 3.1 Å for the minor orientation, respectively. Also, the Cp-THF ether oxygen forms a strong hydrogen bond with the backbone amide NH of Asp29. Without being bound by theory, it is believed that these new interactions with the backbone atoms of Gly48 are responsible for the high antiviral activity against wild-type and drug resistant HIV. The C3-functionality on the Cp-THF appears to enhance the affinity of the inhibitor. Without being bound by theory, it is believed that the new water-mediated interaction with the backbone NH of Gly48 on the protease flap may promote thermodynamic stabilization of the closed conformation of the protease-ligand complex. This interaction may slow the kinetics of dissociation of the inhibitor through flexible opening of the protease flap.
Preparation of C3-substituted hexahydrocyclopentafuranyl urethanes as P2-ligands with enhanced interactions with the protein backbone in the S2-subsite is described herein. The ligands were stereoselectively synthesized in optically active form. Incorporation of these ligands in (R)-hydroxyethylsulfonamide isosteres resulted in a series of new and highly potent HIV-1 protease inhibitors. In particular, inhibitor 25 displayed remarkable enzyme inhibitory and antiviral potency. Also, inhibitor 25 has shown excellent activity against multi-PI-resistant variants compared to other FDA approved inhibitors. A protein-ligand X-ray structure of 25-bound HIV-1 protease was determined at 1.45 Å resolution. The inhibitor appears to make extensive interactions throughout the active site. Of particular interest, the 3-(R)-hydroxyl of the Cp-THF ligand formed a new water-mediated hydrogen bond interaction with the backbone amide NH of Gly48 and the Cp-THF ether oxygen formed a strong hydrogen bond with backbone amide NH of Asp29. Without being bound by theory, it is believed that extensive interactions with the protein backbone may be responsible for inhibitor 25's high antiviral activity and drug resistance profiles. The design of inhibitors with additional binding to the protein backbone has led to the development of inhibitors characterized by high potency against both wild-type and multi-drug-resistant HIV-1 strains.
Inhibitors were designed to induce additional interactions in the protease active site, especially by targeting the Gly48 backbone bonds in the flap region of the enzyme. All inhibitors were first tested for their enzyme inhibitory potency using the assay protocol developed by Toth and Marshall. Compounds that exhibited high inhibitory activities were further evaluated by in vitro antiviral assays.
Inhibitors having an oxygen functionality on the C3-position of the Cp-THF ligand were evaluated and their respective activities were compared (Table 1).
TABLE 1Enzymatic inhibitory and antiviral activity of inhibitors.KiIC50Inhibitor(nM)(μM)a0.95 0.014 0.0790.025  0.050, 0.077 0.005, 0.007 0.0600.019  0.95,  0.39 0.037 0.005 0.0029 0.0060.036 0.006 0.0034  0.020, 0.16 0.025  .017—aValues are means of at least two experiments. Human T-lymphoid (MT-2) cells (2 × 103) were exposed to100 TCID50s of HIV-1LAI and cultured in the presence of each PI, and IC50 values were determinedusing the MTT assay. The IC50 values of amprenavir (APV), saquinavir (SQV), and indinavir (IDV)were 0.03 μM, 0.015 μM, and 0.03 μM, respectively.
As can be seen from the results shown in Table 1, all inhibitors displayed sub-nanomolar to low picomolar inhibitory potencies. Inhibitor 22, with a 3-(S)-hydroxy group on the Cp-THF, displayed high inhibitory potency of 50 pM and a high antiviral activity with an IC50 of 5 nM (Table 1). Inhibitor 23 also displayed high enzyme inhibitory potency and cellular activity. However, the IC50 was 4-fold higher than that of 22. Inhibitor 19b that contains 3-(S)-methoxy substituent, exhibited loss of potency (Ki=0.95 nM) and a near 7-fold loss of antiviral activity (IC50=37 nM) compared to 22.
Compared with inhibitor 25, inhibitor 26 with the 4-amino sulfonamide isostere showed slight loss of activity. Inhibitor 19d, with a 3-(R)-methoxy group, provided a comparable inhibitory potency (6 pM) and antiviral activity. These results contrast with those obtained with inhibitor 22 and 19b.
Inhibitors 19e and 19f, with a methyl group in place of a C3-oxygen substitution, were synthesized to understand the importance of the C3-hydroxyl oxygen on the Cp-THF ring in 22 and 25. Inhibitor 19e, which contains a (R)-methyl group in place of the (R)-hydroxyl on the C3-position showed a 5-fold lessening of antiviral activity compared to 22. A similar lessing of activity was also observed with inhibitor 19f, confirming the critical role of the cyclic THF oxygen for the inhibitor potency.
Inhibitors containing CP-THF ligand with amine substitutions were also prepared. Inhibitors with C3-N-amide, -sulfonamide or a free amine on the Cp-THF core were synthesized and the inhibitors were evaluated for their enzymatic potencies and antiviral activities (Table 2).
TABLE 2Enzymatic Inhibitory and Antiviral Activity of Inihibitors.KiID50Inhibitor(nM)(μM)0.0074 0.0215, 0.025  0.00750.031   0.017,  0.032 0.028  0.18 0.05  0.00180.0016 0.004 0.0046 0.088 0.0815 —— 12.7 nM— 0.020 0.0047
As shown in Table 2, inhibitors 19g and 19h with an acetamido group on the P2 ligand displayed high enzymatic potency (both 7 pM) and antiviral activities (IC50=25 nM and 31 nM, respectively). The opposite stereochemistry at C-3 for inhibitors 19i and 19j had little influence on the respective activities, which were comparable.
Inhibitors 22 and 25 were tested against a panel of multidrug-resistant HIV-1 variant and its antiviral activity with that of other clinically available PIs including DRV. Results are shown in Table 3. All tested inhibitors exhibited high antiviral activity against the wild-type HIV-1 laboratory strain, HIV-1ERS104pre, isolated from a drug-naïve patient. Compound 25 provided the most potent activity with an IC50 of 2.9 nM, comparable to that of DRV (IC50=3.7 nM). When tested against various multidrug-resistant HIV-1 strains, 25's IC50 remained in the low nanomolar values (i.e. 2.9-29 nM), and fold-change in IC50 did not even exceed 10. Interestingly, when compared to 25, isomeric inhibitor 22 displayed lower activity against the wild-type viral strain (IC50=20 nM). It also exhibited much larger IC50 fold-change, or even negligible activity, against multidrug-resistant HIV-1. Such a stark contrast in antiviral activities underlines the importance of the stereochemistry at the 3-position of the P2 ligand. Clinically available PI, APV, exhibited lower IC50 values and smaller resilience to drug-resistant viruses. Overall, inhibitor 25 maintained impressively high activity against all multidrug-resistant HIV-1 strains. It compared favorably with DRV, which is the leading PI for the treatment of multi-drug resistant HIV infection.
Inhibitors 19l and 19m, were further evaluated against a panel of multidrug-resistant (MDR) HIV-1 variants and their antiviral activities were compared to clinically available PI, darunavir (DRV). Results are shown in Table 4. All inhibitors exhibited low nanomolar EC50 values against the wild-type HIV-1ERS104pre laboratory strain, isolated from a drug-naïve patient. Inhibitor 3d had the most potent activity (EC50=3 nM) similar to that of DRV. When tested against a panel of multidrug-resistant HIV-1 strains, the EC50 of 19m remained in the low nanomolar value range (15-24 nM) and its fold-changes in activity were similar to those observed with DRV. Interestingly, inhibitor 19l, with the opposite (S)-stereochemistry at C3, displayed slightly lower antiviral activities against all viral strains compared to 19m. However, the fold changes in EC50 for 19l remained low (<3) against all MDR HIV-1 viruses. The fold-changes contrasted with those of 19m and even DRV, for which the respective EC50's increased by a factor of at least three against the MDR viruses examined.
TABLE 3Comparison of the antiviral activity of 22, 25, and of other PIs against multidrug resistant HIV-1 variants.IC50 (μM) ± SDs, (fold change)bVirusa2522APVDRVHIV-1ERS104pre (wild type)0.0029 ± 0.00080.020 ± 0.0040.030 ± 0.0060.0037 ± 0.0001HIV-1MDR/B 0.029 ± 0.007 (10)>1 (>50) 0.93 ± 0.28 (31) 0.036 ± 0.013 (10)HIV-1MDR/C 0.022 ± 0.003 (7)>1 (>50) 0.26 ± 0.03 (9) 0.013 ± 0.0004 (4)HIV-1MDR/G0.0045 ± 0.0007 (2) 0.27 ± 0.02 (13) 0.38 ± 0.03 (12)0.0023 ± 0.0006 (1)HIV-1MDR/TM0.0031 ± 0.002 (1)0.041 ± 0.004 (2) 0.19 ± 0.06 (6)0.0019 ± 0.0003 (1)aAmino acid substitutions identified in the protease-encoding region compared to the consensus type B sequence cited from the Los Alamos database; L10I, L33I, M36I, M46I, F53L, K55R, I62V, L63P, A71V, G73S, V82A, L90M, and I93L in HIV-1MDR/B; L10I, I15V, K20R, L24I, M36I, M46L, I54V, I62V, L63P, K70Q, V82A, and L89M in HIV-1MDR/C; L10I, V11I, T12E, I15V, L19I, R41K, M46L, L63P, A71T, V82A, and L90M in HIV-1MDR/G; L10I, K14R, R41K, M46L, I54V, L63P, A71V, V82A, L90M, I93L in HIV-1MDR/TM. HIV-1ERS104pre served as a source of wild-type HIV-1. IC50s were determined by using PHA-PBMs as target cells, and inhibition of p24 Gag protein production by each drug was used as an end point. Numbers in parentheses represent n-fold changes of IC50s for each isolate compared to IC50s for wild-type HIV-1ERS104pre. All assays were conducted in duplicate or triplicate, and data shown represent mean values (±1 standard deviation) derived from results of three independent experiments. PHA-PBMs were derived from a single donor in each independent experiment. DRV (Darunavir), APV (Amprenavir).
TABLE 4Comparison of the antiviral activity of 19l, 19m, and DRV againstmultidrug-resistant HIV-1 variantsEC50 (μM) ± SDs, (fold-change)Virusa19l19mDRVHIV-1ERS104pre0.029 ± 0.0020.003 ± 0.0010.004 ± 0.001(wt)HIV-1MDR/B0.075 ± 0.011 (3)0.018 ± 0.003 (6)0.019 ± 0.006 (5)(X4)HIV-1MDR/C0.030 ± 0.006 (1)0.015 ± 0.005 (5)0.011 ± 0.003 (3)(X4)HIV-1MDR/G0.039 ± 0.001 (1)0.020 ± 0.005 (7)0.011 ± 0.002 (3)(X4)HIV-1MDR/TM0.074 ± 0.006 (3)0.024 ± 0.004 (8)0.028 ± 0.001 (7)(X4)aAmino acid substitutions identified in the protease-encoding region of HIV-1ERS104pre, HIV-1MDR/B, HIV-1MDR/C, HIV-1MDR/G, HIV-1MDR/TM compared to the consensus type B sequence cited from the Los Alamos database include L63P in HIV-1ERS104pre; L10I, K14R, L33I, M36I, M46I, F53I, K55R, I62V, L63P, A71V, G73S, V82A, L90M, I93L in HIV-1MDR/B; L10I, I15V, K20R, L24I, M36I, M46L, I54V, I62V, L63P, K70Q, V82A, and L89M in HIV-1MDR/C; L10I, V11I, T12E, I15V, L19I, R41K, M46L, L63P, A71T, V82A, and L90M in HIV-1MDR/G; L10I, K14R, R41K, M46L, I54V, L63P, A71V, V82A, L90M, I93L in HIV-1MDR/TM. HIV-1ERS104pre served as a source of wild-type HIV-1.bEC50 values were determined by using PHA-PBMs as target cells and the inhibition of p24 Gag protein production for each drug was used as an endpoint. The numbers in parentheses represent the fold-change in EC50 values for each isolate compared to the EC50 values for the wild-type HIV-1ERS104pre. All assays were conducted in duplicate, and the data shown represent mean values (±1 standard deviations) derived from the results of two or three independent experiments. PHAPBMs were derived from a single donor in each independent experiment. DRV (darunavir).Chemistry
The synthesis of 3-keto and 3-(S)-methoxy Cp-THF ligands is shown in Scheme 1. Optically active alcohol 6 was prepared in multigram quantities as described previously. (Deardorff, et al., Org. Synth., Coll. Vol. IX, 1998, 9, 487) This was efficiently converted to ketone 7 as reported previously (Ghosh, et al., Org. Lett. 2008, 10, 5135-5138). The removal of TBS-ether by exposure to HF-pyridine afforded keto alcohol 9a in 94% yield. Ketone 7 was converted to 3-(S)-methoxy derivative 9b in a three-step sequence involving (1) reduction of the ketone with NaBH4 in ethanol at −23° C. to provide the corresponding alcohol as a single diastereomer; (2) methylation of the resulting alcohol with MeI in the presence of Ag2O in acetonitrile; and (3) removal of the silyl group with TBAF in THF to provide 9b in 66% yield, in 3 steps.
The syntheses of 3(R)-acetoxy and 3(R)-methoxy ligands 9c and 9d are outlined in Scheme 2. Treatment of alcohol 6 with NaH and 2-bromoacetic acid in THF provided the corresponding alkylated acid. The resulting acid was reacted with methyl iodide in the presence of NaHCO3 to provide methyl ester 10 in 53% yield (2 steps). Dibal-H reduction of ester 10 followed by radical cyclization of the resulting alkene using a catalytic amount of (nBu3Sn)2O, Ph2SiH2 and ethanol (2 equiv) in the presence of a catalytic amount of AIBN in benzene provided 3-(R)-hydroxy derivative 11 in 64% yield, in two steps. The 1H-NMR analysis showed a diastereomeric ratio of 10:1. The major isomer was separated by silica gel chromatography and used for the subsequent reactions. Reaction of alcohol 11 with acetic anhydride and triethylamine in the presence of a catalytic amount of DMAP afforded the corresponding acetate. The removal of silyl group with TBAF in THF provided ligand 9c in 73% yield, in 2 steps. Alcohol 11 was converted to methoxy derivative 9d by alkylation with NaH and MeI in THF followed by removal of the silyl group in 71% yield (2 steps).
In another aspect, synthesis of stereochemically defined 3-methyl derivatives to compare the effects of alkoxy and hydroxy groups is described. Stereoselective syntheses of 3(S) and 3(R)-methyl derivatives is described in the synthetic route shown in Scheme 3. Optically active olefin 113 was synthesized as described previously. Catalytic hydrogenation of 113 in the presence of Wilkinson's catalyst under a hydrogen filled balloon at 23° C. for 3 h, followed by removal of the silyl group using TBAF afforded 3(S)-methyl derivative 9e. For the synthesis of the 3-(S)-methyl derivative, commercially available optically active lactone (+)−115 was methylated using LDA and MeI at −78° C. to provide methyl derivative 116 with high diastereoselectivity (dr=20:1) and in 95% yield. Olefin 116 was then subjected to oxymercuration condition with Hg(OAc)2. The resulting organomercurial derivative was treated with aqueous sodium hydroxide solution followed by NaBH4 reduction to afford endo-alcohol 117 in 64% yield. The lactone was then reduced to the corresponding lactol with Dibal-H. Further reduction of the resulting lactol using Et3SiH and TiCl4 furnished ligand 3-(R)-methyl derivative 9f in 68% yield. The 1H-NMR NOESY experiments fully corroborated the assignment of 3(S)- and 3(R)-stereochemistry of methyl derivatives 9e and 9f, respectively.

Various optically active ligand alcohols 9a, 9b, 9c, 9d, 9e, and 9f were converted to the respective mixed activated carbonates. As shown in Scheme 4, reactions of ligand alcohols with 4-nitrophenyl chloroformate in the presence of pyridine in CH2Cl2 provided activated carbonates 15a-f in 50-96% yield. The synthesis of designed inhibitors was carried out by coupling these activated carbonates with various hydroxyethylsulfonamide isosteres containing functionalized P2′-phenylsulfonamide ligands. As shown in Scheme 5, amines 16-18 were readily prepared as described previously. Reaction of amine 16 with carbonate 15a provided inhibitor 19a. Inhibitors 22, 123 and 23 were prepared by reaction of carbonate 15a with respective amines 16-18 followed by NaBH4 reduction of the resulting ketone derivatives. The inhibitor structures are shown in Table 1. Inhibitors 19b and 19d-f were prepared by reactions of amine 16 with mixed carbonates 15b and 15d-f, respectively. The synthesis of inhibitors 25 and 26 was carried out by reactions of mixed carbonate 15c with amines 16 and 17 followed by removal of the acetyl group with K2CO3 in methanol. All inhibitors were prepared in good to excellent overall (41-96%) yields. The synthesis of inhibitor 24c containing a dimethylamine functionality was carried out by reductive amination of ketone 19a with Me2NH2+OAc− in the presence of NaHB(OAc)3. We have also attempted to prepare the corresponding methylamine derivative by reductive amination with MeNH3+OAc−. However, the resulting 3(S)-methylamine derivative 24b turned out to be unstable.


For the synthesis of the Cp-THF derived ligands, previously reported ketone 725 was used as a common intermediate (Scheme 1). This ketone was easily synthesized in five steps starting from enantiopure alcohol 6, as previously reported (Scheme 1) (Ghosh, et al., Org. Lett. 2008, 10, 5135-5138). Initially inhibitors including oxygen-containing functionalities on the C3-position were prepared. For the syntheses of a 3-(R)-hydroxy Cp-THF ligand, and its corresponding inhibitor, ketoalcohol 9a was used as a precursor (Scheme 1). It was directly obtained from 7 by simple removal of the TBS group with HF-pyridine in up to 94% yield. Ligand 9b containing a 3-(R)-methoxy group was synthesized in three steps, starting with 1) stereoselective reduction of ketone 7 using NaBH4 in EtOH at −25° C., 2) methylation of the free hydroxyl group using MeI/Ag2O, then 3) removal of the silyl group with TBAF to give 9b in 76% yield (3 steps).

The syntheses of the corresponding P2 ligands with opposite (S)-stereochemistry at the C3-position are shown in Scheme 2. Reaction of alcohol 6 with 2-bromoacetic acid, followed by methylation of the resulting acid, furnished methyl ester intermediate 10. DIBAL-H reduction of the methyl ester to the corresponding aldehyde and radical cyclization of the 1,5-alkenal using (nBu3Sn)2O, Ph2SiH2, AIBN and EtOH in benzene, gave desired intermediate 3-(R)-11 in 67% yield (2 steps, d.r.=10:1). For the synthesis of an inhibitor containing a 3-(R)-hydroxy group on the Cp-THF, ligand precursor 9c was made by 1) protection of the free hydroxyl group of 11 with Ac2O and 2) TBAF-mediated deprotection of the silyl group. The 3-(S)-methoxy Cp-THF analog was synthesized by methylation of alcohol 12 followed by TBAF-deprotection to furnish corresponding P2 ligand 9d (Scheme 2).

Ligands 9e and 9f in which the C3-hydroxyl was respectively replaced by a methyl group were also prepared to aid in understanding the gain in binding affinity of provided by the C3 hydroxyl. (Scheme 3a). The synthesis of P2 ligand 9e is outlined in Scheme 3a. Intermediate 6 was reacted with (Z)-1-ethoxy-propene in the presence of N-iodosuccinimide in CH2Cl2 to furnish the corresponding iodoacetal 12, as a ca. 1:1 mixture of diastereoisomers. Radical cyclization of the 1,5-iodoalkene was performed utilizing n-Bu3SnH and Et3B as a radical initiator to furnish the corresponding cyclized ligand 13 (>99%, four diastereoisomers, d.r.=1:1.7:2:2.7). Reduction of the acetals under Kishi's condition (Et3SiH, BF3-Et2O) followed by TBAF-promoted removal of the TBS group provided the respective 3-(S) and 3-(R) diastereoisomeric alcohols. Ratio and assignment of stereochemistry of 9e and 9f were subsequently confirmed by NOESY analysis on their respective mixed activated carbonate.

As mentioned above, X-ray crystal structure of GRL02310-bound HIV-1 protease revealed a close proximity of the Cp-THF C3 methylene to the backbone carbonyl of Gly-48 in the flap region of the protease. Several inhibitors containing a hydrogen bond donor, such as the NH of an amide, sulfonamide, or carbamate in that position of the P2 ligand were prepared, to determine the effect of a hydrogen bond with the Gly-48 carbonyl. Without being bound by theory, it is believed that the carbonyl or sulfonamide oxygens could also engage in hydrogen bonding with the proximal Gly-48 NH backbone bond. To complete the synthesis of ligands containing a nitrogen-based substituent, ketone 7 was modified into O-methyl oxime 14 in 96% yield (Scheme 4a). The oxime was then reduced to the corresponding amine under hydrogenative conditions using a mixture of Pd/C and Raney Ni to give the corresponding amine as a 3:1 mixture of diastereoisomers.

Following reduction of oxime, the crude amine was directly treated with Ac2O in presence of Et3N and a catalytic amount of DMAP giving the corresponding isomeric TBS-protected amide intermediates. Treatment of the respective amides with TBAF in THF furnished isomeric ligands 9g and 9h. Likewise, the amines mixture was reacted with MsCl and Et3N and subsequent treatment with TBAF furnished the diastereoisomeric sulfonamides 9i and 9j, respectively. For comparison purposes, ligand 9k, which contains a 3-(R)-dimethylaminoformamoyloxy group at the C3 position, was also synthesized. This ligand possesses a carbamate carbonyl group as potential hydrogen bonding acceptor but is devoid of the NH bond as hydrogen bonding donor.
A general method for the synthesis of inhibitors is shown in Scheme 5a. Alcohols 9a-k were respectively reacted with para-nitrophenyl chloroformate in the presence of pyridine in CH2Cl2 to furnish the corresponding activated carbonates 15a-k. The syntheses of the HIV protease inhibitors were then carried out as shown in Scheme 5a. Hydroxyethylene isosteres 16, 17, and 18, were respectively treated with 30% TFA in CH2Cl2 to furnish the corresponding deprotected amine isosteres. The amines were reacted with the appropriate mixed activated carbonate, 15a-m, in presence of Et3N in THF/CH3CN at 23° C. for 2-4 days to furnish the corresponding inhibitors, or intermediates, 19a-k, 20, and 21.

Ketone-based inhibitor intermediates 19a, 21 were obtained from the coupling of 15a with isosteres 16, and 18, respectively. Both were reacted with NaBH4 in EtOH to respectively furnish inhibitors 22, and 23, that contain the 3-(S)-hydroxy Cp-THF ligand (Scheme 6).

Synthesis of inhibitors with a free amine functionality was undertaken. Thus, compound 19a was reacted with O-methyl hydroxylamine hydrochloride to form the corresponding oxime, which was reduced with NaBH3CN in acetic acid to give O-methyl hydroxylamine inhibitor 24a. Reductive amination of ketone 19a with MeNH2 and Me2NH provided the respective inhibitors 24b and 24c.
The acetate of each intermediate 19c and 20 was removed by methanolysis. The reactions furnished respective free 3-(R)-hydroxyl-containing inhibitors 25 and 26 (Scheme 7).
